To protect metal surfaces against corrosion, it is known to use methods in which a coating of a different and, as a rule, less noble metal (e.g., zinc and chromium) is applied to the metal surface. Thus, it is known, e.g., to coat steel with zinc or chromium or even with tin (which is, however, more noble than steel). In the production of packagings, in particular in the food industry, for example, tin-coated black plate (tinplate) is very extensively used. Tinplate has excellent corrosion resistance and good formability and weldability, which makes it highly suitable for use in the production of packagings, e.g., beverage cans.
In order to also protect the metal coating, for example, the tin coating for tinplate, against corrosion and to create a good base surface for paint and plastic coatings, conversion coatings are frequently applied to the surface of the metal coating.
Conversion coatings are defined to mean very thin, in most cases, inorganic, metal coatings on a metal surface, which, as a rule, are created by chemical reaction of an aqueous treatment solution with the metal substrate. In the no-rinse process, for example, these conversion coatings are applied by means of a roll coater or a spray coating system. Especially on black plate, conversion coatings ensure a highly effective protection against corrosion and a good base surface for paint and plastics, and they reduce surface friction and abrasion.
Depending on the substrate, a distinction is made between iron, zinc or manganese phosphating, electrolytic phosphating and chromate, oxalate and anodizing processes. Chromium-containing conversion coatings have been demonstrated to be highly effective in protection against corrosion. During chromating, the metal surface is treated with an acid solution containing chromium(VI) ions, in the course of which chromium(VI) is reduced to chromium(III). As a result of the treatment, a chromium-containing anti-corrosive coating forms on the metal surface.
Chromium(VI) compounds are, however, acutely toxic and carcinogenic. In the EU, the passivation of metal surfaces with chromium(VI)-containing substances has already been prohibited for use in the manufacture of automobiles and household appliances. For this reason, chromium-free conversion coatings have been developed in the state of the art. Thus, for example, methods of producing chromium-free conversion coatings on zinc and aluminum surfaces are known from WO 97/40208-A and EP 2532769 A1. Furthermore, WO 2008/119675 describes treatment solutions for producing chromium-free conversion coatings which contain oxo cations and complex halogen ions, which lead to colorless and slightly iridescent conversion coatings.
Tinplate has excellent properties as a packaging material for food products and has been produced and processed for many decades for this purpose. However, because of the global shortage of this resource, tin, which in tinplate is the corrosion-inhibiting coating, has become a relatively expensive material. As an alternative to tinplate, it is known from the prior art that it is possible to use electrolytically chromium-coated steel, especially for use as packaging steel, which is referred to as “Tin Free Steel,” (TFS) or as “Electrolytic Chromium Coated Steel (ECCS).” On the one hand, these tin-free steels provide excellent adhesion for paints or organic protective coatings (for example, made of PP or PET), but, on the other hand, because of the toxic and harmful properties of the chromium(IV)-containing materials used in the coating, there are considerable disadvantages to using them in the process of applying the coating.
These disadvantages can be avoided when using the method of passivating black plate in coil form as known from DE 10 2013107506 A1. DE 10 2013107506 A1 offers the possibility of passivating black plate without the use of chromium-containing treatment solutions and thereby protecting it against corrosion. Black plate treated according to this method can be used as a substitute for tinplate and tin-free steel (TFS or ECCS), for example, in the production of metal packagings, such as cans. For use of black plate described in DE 10 2013107506 in the production of cans, the passivated black plate is coated at least on one surface with an organic coating, e.g., paint or polymer coatings made of PET, PP or PE or combinations thereof so as to improve the corrosion resistance. In the production of cans, the coated side forms the inside surface of the can which may come into contact with acidic components and must therefore be especially well protected against corrosion, but it is also possible to coat both sides so as also to protect the outside surface of the can against corrosion in a humid atmosphere.
However, it was found that organic coatings on this black plate do not adhere sufficiently well to the passivated black plate surface. Especially if the drying time is short enough to be measured only in seconds, as is standard in conventional coil coating processes in which black plate in the form of a coil moves at a coil rate of more 200 m/min, it was found that the organic coatings (paint or polymer coating) on the black plate surface passivated with a conversion coating do not sufficiently adhere to black plate during the subsequent forming process (for example, in deep drawing processes for the production of cans). Comparative experiments have shown that under the mechanical loads that prevail during forming processes, it is possible for the conversion coating to become detached from the steel surface of black plate. To improve the adhesion of organic coatings to the black plate surface which has been passivated with the conversion coating, DE 10 2013107506 A1 proposes that a bonding agent be used, which bonding agent is mixed into the treatment solution which is applied to the black plate surface to create the conversion coating. However, this is able to only improve the adhesion of the organic coating to the conversion coating, but does not prevent the conversion coating from detaching from the black plate surface when the mechanical load is high.